Impact of Alternative Propulsion Systems on Contrail Formation and Lifetime

Abstract

The reduction in aviation-induced climate impacts requires not only decreasing CO2 emis sions, but also mitigating non-CO2 effects such as persistent contrail cirrus. Alternative propulsion systems and fuels are therefore discussed as potential pathways toward climate neutral aviation. However, their influence on contrail formation and evolution remains insufficiently understood, particularly regarding how emission characteristics translate into contrail microphysical behavior and lifetime. This study aims to bridge this knowledge gap by systematically evaluating the contrail-forming potential of Sustainable Aviation Fuel (SAF) and hydrogen-based propulsion systems, with a clear focus on identifying the key emission parameters that govern contrail persistence. To assess the impact of reduced particle emissions on contrail evolution, scenarios with lower initial ice crystal number concentrations Nice, representative of alternative propulsion concepts, were simulated using a Gaussian plume contrail evolution model. The results demonstrate that high ice crystal number concentrations (Nice ≈ 1014–1015 kg−1), characteristic of kerosene and many SAF combustion cases, lead to persistent contrails with lifetimes of approximately 9–12h due to suppressed crystal growth and sedimentation. Conversely, low initial ice crystal con centrations (Nice ≈ 109–1010 kg−1), expected for hydrogen systems under aerosol-limited nucleation conditions, promote short-lived contrails in the order of minutes. The findings indicate that contrail impacts are influenced not only by the fuel type itself, but also by combustion processes, atmospheric conditions, and the prevailing nucleation mechanisms. Consequently, the potential of SAF and hydrogen propulsion systems to mitigate contrail related climate effects will likely depend on further technological optimization as well as additional experimental and in-flight observations to better quantify their atmospheric impacts. This work underscores the importance of linking emission measurements di rectly to contrail microphysics and provides a framework for evaluating future propulsion technologies based on their actual contrail-forming potential, rather than solely on fuel composition or CO2 reduction.